For many years heat-sensitive imaging materials have been used for copying, thermal printing, thermal recording and thermal labeling. Typically, thermal imaging with these materials involves thermally increasing the reactivity of two or more components of a color forming reaction which do not react at normal ambient temperatures. Reactivity is often enhanced by melting one or both reactants which are physically separated from one another. Generally, physical separation of the color forming components is accomplished by either situating them in separate coated layers or by dispersing them in a single coated layer.
In commercial thermal labeling or "bar-coding" applications, thermally developed labels are sought which have thermally generated, light-stable images, capable of being read or scanned by image scanning devices (scanners), disposed on intensely colored background media. In addition to the aesthetic appeal of such labels, they are highly desirable as a means for providing easy visual differentiation, on the basis of color, between labeled items. Such labels can be used to differentiate between labeled items with respect to a particular feature such as production date, product size, product line, make, model, etc., simply by associating said feature with a particular background color. Black is especially desirable where it is necessary to prevent visual detection of the image for security purposes.
The ability of a scanner to scan an image, or to discriminate between the image and the background, is measured by the "print contrast signal". The print contrast signal is measured and expressed as a function of the particular wavelength of radiation the imaged media is exposed to, and is defined as the quotient of the difference between the background reflectance and the image reflectance divided by the background reflectance, i.e. [(R.sub.bkg -R.sub.image)/R.sub.bkg ]. The greater the print contrast signal the easier the imaged media can be scanned by a scanner. In this regard, the Uniform Product Code Council has issued performance standards for scanner scannable imaged media which establish the maximum allowable image reflectance for any given value of background reflectance according to the following algorithm: EQU log.sub.10 R.sub.image =2.6 (log.sub.10 R.sub.bkg)-0.3
Thus for any measured value of background reflectance, the maximum allowable image reflectance, and hence, the minimum allowable print contrast signal, can be determined. At print contrast signal values below the calculated minimum, the "first scan rate", a measure of the accuracy of correctly reading the image on the first scan, will be unacceptably low. At print contrast signal values above the calculated minimum, the scanner can quickly and accurately scan the image on the first scan.
Increasing the first scan rate of imaged media by a scanner may thus be accomplished by increasing the difference between the background reflectance and the image reflectance at the specific wavelength of radiation used in the scanner. To achieve high scanner first scan rates at any specific wavelength it is generally preferred that the imaged area absorb strongly (low reflectance) at that wavelength and the background absorb weakly (high reflectance) at that wavelength.
The most important wavelengths of radiation for bar-coding applications are determined by the scanning equipment commercially available. Scanners scanning in the visible region of the electromagnetic spectrum typically employ a helium-neon laser and read at 633 nm. Scanners scanning in the near infrared spectral region typically employ a gallium-arsenide laser diode and read at 905 nm. Recently, however, a new spectral source has become available for scanning in the near infrared region which enables an scanner to scan at 850 nm.
When an image is scanned by a scanner, it is essential that the background not interfere with the reading of the imaged areas. There must be clear and distinct contrast between the imaged and non-imaged areas for the scanner to be able to distinguish between the two. For this reason, thermographic labels intended to be scanned by scanners operating in the visible region of the electromagnetic spectrum typically have white or very lightly colored backgrounds. Lightly colored media, such as light pinks and yellows, have been used and found acceptable for scanning in the visible region at a wavelength of 633 nm, since these lightly colored materials absorb too weakly at that wavelength to interfere with the reading of the imaged areas. The use of intensely colored thermographic media in conjunction with scanners scanning in the visible region, on the other hand, has generally been proscribed, despite the above-mentioned desirability of such labels in the marketplace, because the background reflectance is generally too low to provide acceptable first scan rates.
For thermographic labels intended to be scanned in the near infrared spectral region, it is likewise essential that the image and the background differ sufficiently in their absorptance at the wavelength being used, to provide sufficient contrast for the scanner to distinguish between the two. It is generally preferred that the image absorb strongly in the near infrared and the background reflect strongly in the near infrared in order to provide such image discrimination.
An example of such a thermographic label has been commercially available from the Minnesota Mining and Manufacturing Company, since 1980, under the trade designation "Scotchmark" brand thermal label stocks. These labels provide a thermally generated image which absorbs strongly in both the visible and near infrared spectral regions by reacting an iron salt with a catechol to form a near infrared absorbing complex. The image is disposed on a background colored a light yellow by the dispersion of yellow pigment throughout the reactant containing layer of the label. However, despite the presence of the yellow pigment, these labels have sufficient contrast between the image and the background to meet the Uniform Product Code Council performance standards for scanning in both the visible and the near infrared spectral regions. There thus appears to be a presumption in the industry that the background should be lightly colored or white to avoid interfering with the image scannability even when scanning in the near infrared spectral region.
Contrary to this presumption, the inventor of the present invention has discovered that infrared scannable thermal labels can have intensely colored backgrounds while maintaining sufficient image discrimination to meet the Uniform Product Code Council performance standards for scanning in the near infrared spectral region. These labels provide the industry with the aesthetic appeal and color coding ability it desires. Furthermore, such intensely colored labels provide the industry with the ability to prevent visual discrimination of the image by increasing the intensity of the background color, where desirable for security reasons, without prohibiting scanning of the image with a scanner.